Re: Custom built, legal and self-propelled 'car'
Since you are here (welcome by the way) I'm guessing you're thinking in terms of something light weight and on the small side. First, if you are thinking about 4 wheels then you will be subject to a whole lot of scrutiny by the motor vehicle people and subject to a lot of hassle. Something with three wheels however is another ball game as it is considered a motorcycle. That is the way to go if you want to be street legal... unless you have very deep pockets and are willing to build something the powers that be will approve.
I'd suggest you look up a thread called Vellocars and other interesting vehicles. There are a number of us here who are interested in building three wheeled alternative vehicles. Ricks Rides is building one now, Intrepid Wheelwoman is putting the parts together for an upcoming cylclecar with two wheels in the back and one up front, while Fasteddy and I are planning our builds in the tadpole configuration with two wheels up front, and one in back.
Wander around the site a bit. There's a section for trikes here which also has some interesting material for your perusal. Good luck with your project and when you do start something share it here with a build thread. I know that the interest in cyclecars is growing. You are among friends!
Someday when I grow up I will probably lose interest in toys with wheels, but until then...